In typical liquid-cooled compressors, liquid is injected in a compression stroke for the purpose of lubrication, sealing, and cooling. Since any droplet must not be contained in compressed air supplied from an air compressor, each liquid-cooled compressor is provided therein with a separator for separating compressed air and liquid from each other. Liquid separated at the separator is trapped at the lower part of the separator. Utilizing a pressure difference between the separator and the compressor body, the liquid is then injected into the compressor body and a bearing by way of a heat exchanger and a filter and lubricates and cools male and female rotors and a bearing.
For this reason, in a capacity control state in which reduced pressure operation is performed under no load, it is necessary to slightly open an intake valve to take in liquid and compress the liquid to a predetermined pressure (hereafter, referred to as reduced pressure operation pressure P1, P1>P2) in order to maintain a pressure difference (hereafter, referred to as minimum circulation oil supply pressure P2) at which liquid can be recovered into the compressor body and inject the liquid into a bearing and the like to lubricate and cool the same for ensuring reliability. This poses problems that surplus compression power is required and energy efficiency during no-load operation is degraded.
To solve the above problems, a technique in which a compressor is stopped under no load is known. However, when a large-output electric motor is frequently started and stopped, heat in the electric motor is not dissipated and a probability of occurrence of coil burnout or the like is increased. This poses a problem of regraded compressor reliability.
As a background art in the present technical field, there is Japanese Patent No. 3262011 (Patent Literature 1). Patent Literature 1 relating to a screw compressor equipped with a revolving speed controller is based on a technology in which the compressor is automatically started and stopped during capacity control operation to reduce power during no-load operation. Before the compressor is automatically stopped, compression is performed until the pressure is made higher than a specified pressure. Stopping durations are thereby lengthened to prevent increase in a number of stopping times.